The objective of the research is to define further the central organization of the somatosensory systems with relation to their role in the perceptual basis of behavior. Specifically, the research aims to determine how descending influences arising in higher brain centers may modify the activity of ascending somatosensory pathways, which include the spinocervical, spinothalamic, spinoreticular and rostral spinocerebellar tract systems. An electrophysiological approach will be employed to identify and study the responses of ascending spinal tract neurons of the cervical enlargement in both acutely anesthetized and chronic, behaving animals. The planned experiments include the following: (1) determination of the effect of descending influences from cerebral cortex and brainstem-vestibular structures on responses of ascending spinal neurons evoked by natural peripheral stimuli, including analyses of inputs to the descending systems themselves; (2) investigation of the central and peripheral influences modulating activity of somatosensory neurons in conjunction with changes in postural orientation produced by static tilt in decerebrate cat; (3) study of central and peripheral influences on brainstem lemniscal neurons during passive and active voluntary movements in chronic, unanesthetized monkeys. BIBLIOGRAPHIC REFERENCES: Brown, A.G., Coulter, J.D., Rose, P.K., Short, A.D. and Snow, P.J. 1977 Inhibition of spinocervical tract discharges from localized areas of the sensorimotor cortex in the cat. J. Physiol. (Lond.) (in press). Peterson, B. W. and Coulter, J. D. 1977 A new long spinal projection from the vestibular nuclei in the cat. Brain Res., 122: 351-356.